The use of phase locked loops (PLLs) in the fields of cable television (CATV), TV tuning, AM/FM radios, scanning receivers, two way radios, amateur radios, etc. is well known. A typical PLL basically consists of a phase detector, a low-pass filter and a voltage controlled oscillator (VCO). These enumerated elements are coupled to form a closed-loop system. The phase detector measures differences in phase between a reference frequency and a feedback signal frequency from the VCO. Any detected differences generate an error signal which is filtered and supplied to the VCO. The VCO utilizes the error signal so as to minimize the frequency difference between the feedback signal and the reference signal.
A frequency synthesizer is usually made as an IC chip, and must have several output features in order that one generic version of the IC chip may meet or may be adapted to meet any application requirements. A user can select and use one or more desired outputs depending on the application. Howevr, because all output pins are always providing output signal values, unused output pins generate noise causing electromagnetic interference (EMI) and radio frequency interference (RFI) which badly affects other external circuit devices and other elements inside the IC chip.
In order to avoid such a noise problem, it is known to use wire bonding options in which unnecessary output pads in the IC chip are not bonded to pins. However, in this solution, the unnecessary output pads still provide their outputs and badly affect other elements in the IC chip. Another known technique is to use metallization options in which the unused output devices are not connected to pads or other elements in the IC chip. In this case, however, different masks must be prepared to produce different types of chips. In both cases, an IC chip is locked into a specifically configured package that only has the desired outputs, and such a specific IC chip cannot readily be used in a variety of applications. This seriously limits the use of the chip and requires that many different types of chips be made.